1. Field of the Invention
This invention relates to a method and apparatus for detecting contaminants, and in particular, heavy metal contaminants. More specifically, the invention relates to removing a substance from a liquid and colorimetrically detecting the substance.
2. Background of the Invention
Contamination of the environment has been increasing steadily for years as the use of metals, chemicals, pesticides, and bacterial organisms has increased. Even though the toxicity of various metals has been known for centuries, it is only recently that there has been a serious increase in interest in minimizing human exposure to such metals. Current public awareness of such pollutants and their associated hazards has created a consumer demand for products that are capable of determining the presence of unwanted and potentially dangerous materials.
Some of the more toxic metals include lead, cadmium, mercury, barium, chromium and beryllium. Lead, in particular, has been subject to much attention due to its presence in articles or paints commonly found in the home. See, for example, U.K. Patent Application No. 2 025 047 A; "A Simplified Method for Detection of Lead Contamination of Soil" by J. Preer and G. Murchison, Jr., Environmental Pollution (Series B), vol. 12, pp. 1-13; and "A Spot Test for Detection of Lead in Paint" by J. Sayre and D. Wilson, J. Pediatrics, vol. 46, pp. 783-785 (1970).
As some of the prior art publications indicate, there is a recognized need in the industry for a simple test or method for determining the presence of lead. However, as will become apparent from the remaining descriptions of the prior art, prior to the present invention, an effective and simple test for lead or other metals in liquid samples had not been developed.
In a well known prior art method of detecting lead in paint, sodium sulfide (Na.sub.2 S) is reacted with lead to form lead sulfide (PbS), a black precipitate. The presence of lead is thus confirmed by the appearance of the black precipitate, lead sulfide. This method has several disadvantages: (1) the sodium sulfide is potentially toxic, especially to young children; (2) the black precipitate is difficult to see on dark surfaces; (3) the sodium sulfide releases volatile hydrogen sulfide (H.sub.2 S), which has a noxious odor; and (4) the reagents react with many cations to form black precipitates and thus tend to give false readings on many surfaces.
Another common analytical reagent is a metal complexing agent, rhodizonic acid. For over forty years, rhodizonic acid and salts thereof have been used as analytical reagents to detect heavy metals, including lead, in both qualitative and quantitative analyses. The methodology for using rhodizonate dye is based on two types of tests:
(1) a quantitative determination of heavy metals in solutions using a spectrophotometer to obtain quantitative information; and PA1 (2) qualitative determinations which use filter papers impregnated with the reagent.
In addition, semi-quantitative information can be derived from the use of columns packed with silica gel impregnated with rhodizonate dye. See U.K. Patent Application No. 2 025 047 A.
The Macherey-Nagel Company (Duren, Germany) manufactures a test paper for the determination of lead under the trademark PLUMBTESMO. The PLUMBTESMO strips comprise a heavy filter paper with a reagent impregnated therein. To test for lead in a solution, a strip is dipped into the solution, and observed for a color change that indicates the presence of lead. The PLUMBTESMO strips can also be used to detect lead deposits in motor vehicle tailpipes. However, the PLUMBTESMO strips suffer from several disadvantages. First, the chemicals on the strips rub off on the user's hands and clothes after the reaction takes place, causing contamination of other surfaces and requiring constant clean-up. Second, when attempting to use the strips in solutions, other metals interfere with the reaction, potentially causing false results when testing for lead.
Detection of lead in water is generally accomplished by sending a sample to a testing laboratory where the lead content of the sample is determined by analytical instrumental methods, such as atomic absorption spectroscopy, inductively coupled plasma or anodic stripping voltammetry. These instrumental methods are expensive and require sophisticated users.
For example, one method advanced for detecting trace metals in liquid samples involves preparation of a liquid sample, oxidation of organic matter in the sample by boiling with potassium persulfate, treatment of the sample with ammonium pyrrolidinecarbodithioate, filtering the sample and then analyzing the sample by x-ray spectrometry. This process, described in Tisuo et al., "Preconcentration of Submicrogram Amount of Metals from Natural Waters for X-ray Energy Spectrometric Determination Using Pyrrolidinecarbodithioic Acid", Anal Chem., 57:82-87 (1985), is inaccessible to the average person since the particular equipment required is not available. Moreover, the x-ray spectrometry is extremely sensitive to contaminant metals which may be introduced by the oxidizing agent. In this case, ultra-pure chemicals must be manufactured in order to avoid contamination. Finally, the test described in Tisuo et. al. requires heating the persulfate in order to oxidize the organic matter in the sample, which is disadvantageous in a home use test.
A different method of detecting trace metals in liquid is described in Lo et. al., "Solvent Extraction of Dithiocarbamate Complexes and Back-Extraction with Mercury(II) for Determination of Trace Metals in Seawater by Atomic Absorption Spectrometry", Anal Chem., 54:2536-2539 (1982). This procedure involves the extraction of metal-dithiocarbamate complexes into chloroform followed by back-extraction with a dilute mercury solution. This method involves extremely hazardous chemicals and requires monitoring and controlling the pH levels of each solution utilized. Moreover, this method is not available to the average person due to the complexity of the process, the chemicals used and the equipment needed to conduct the x-ray spectrometry.
Colorimetric methods for the specific determination of a substance such as lead in a liquid such as water have heretofore been unavailable because of the sensitivity required. "A Simple Direct Estimation of Ultramicroquantities of Lead in Drinking Water Using Sodium Rhodizonate" by E. Jungreis and M. Nechama, Microchemical Journal, vol. 34, pp. 219-221 (1986) describes a test which can only detect lead in amounts above about fifty parts per billion. This test involves a number of steps, including preparation of a reagent test strip, heating a solution to dryness and development of the test spots. The reagents used include nitric acid and hydrochloric acid, which are not available or widely used by the average person.
Thus, it should be clear that the lead tests known prior to the present invention are not entirely satisfactory. Therefore, there is a need in the art for a test or method for determining the presence of toxic metals, such as lead. Furthermore, there is a need in the art for a simple, easy-to-use test for determining the presence of metals in a liquid sample.